1. Field of the Invention
The present invention relates to an assembling method for an engine timing system to wind a timing chain around respective sprockets of a crankshaft and a camshaft of an engine, and belongs to the field of engine production technologies.
2. Description of the Background Art
A conventional engine timing system related to the present invention includes one type as disclosed, for example, in JP 2007-023846A. As described in this patent publication, an engine is configured to allow a camshaft formed with a cam for opening and closing an intake or exhaust valve to be rotated in interlocked relation to a rotation of a crankshaft. In overhead camshaft type engines where a camshaft is arranged in a cylinder head, a timing chain is provided to interlock a camshaft and a crankshaft. Specifically, a sprocket is fixed to one end of each of the crankshaft and the camshaft, and a timing chain is wound around the respective sprockets of the crankshaft and the camshaft, so that a rotation of the crankshaft is transmitted to the camshaft through the timing chain to allow the camshaft to be rotated in interlocked relation to the rotation of the crankshaft.
For example, this timing chain is assembled in the following manner. A reference mark is provided on each of the sprockets of the crankshaft and the camshaft in a vicinity of a given one of a plurality of teeth thereof, and, in associated relation with respective ones of the reference marks, crankshaft-alignment and camshaft-alignment marks are provided on respective given ones of a plurality of links of the timing chain. Then, the timing chain is wound around the sprockets, while aligning each of the alignment marks with a corresponding one of the reference marks. Through the mark aligning operation, the timing chain is wound around the sprockets in an adequate condition that a given number of the links are arranged between the respective reference marks of the crankshaft sprocket and the camshaft sprocket.
After completion of the timing-chain winding operation, respective phases of the crankshaft and the camshaft are kept in a given relation. Thus, in each cylinder, opening/closing timings of intake/exhaust valves relative to a piston stroke is adequately set so as to ensure a desirable engine combustion cycle, e.g., intake, compression, expansion and exhaust strokes in a four-cycle engine.
More specifically, due to dimensional variations within tolerances for links and pins of the timing chain or teeth of the sprockets, and clearance variations in a mesh zone between the timing chain and each of the sprockets just after completion of the timing-chain winding operation, which is caused by the dimensional variations and/or variations in quality of the winding operation, a camshaft phase relative to a crankshaft phase is likely to have a certain variation and deviate from an optimal phase (i.e., a phase which is perfectly in the given relation with the crankshaft phase), when the crankshaft is rotated after completion of the timing-chain winding operation, to allow the timing chain to conformably fit with each of the sprockets.
This phase deviation causes a deviation in the opening/closing timings of the intake/exhaust valves to deteriorate engine combustion performance. Particularly, in a high-compression engine having a high effective compression ratio, such a deviation is unacceptable depending on a level thereof. The reason is that the deviation is likely to cause a pre-ignition which is a phenomenon that, when a piston is moved to a position close to a top dead center (TDC) of a compression stroke, an air-fuel mixture is auto-ignited before ignition by a spark plug.
Specifically, if the camshaft phase relative to the crankshaft phase is shifted in an advance direction to advance the closing timing of the intake valve, the piston starts a compressing action at a position closer to a bottom dead center (BDC) with respect to a desired position, and thereby an in-cylinder air-fuel mixture is more highly compressed as compared with a desired compression state. Thus, in the high-compression engine where a compression ratio is originally set, for example, at 13 or more, the pre-ignition is likely to occur due to an abnormal temperature rise caused by the adiabatic compression.